As explained in the article “Base stations for mobile telephony (UMTS-FDD) recommendation on the measures”, project of 17 Sep. 2003, published on the Internet website www.environnement-suisse.ch/electrosmog, UMTS technology (for “Universal Mobile Communication System”) is a third generation mobile telephone technology. Each telecommunication operator operates its own UMTS network.
UMTS radiocommunication systems make it possible to transmit data at rates that are clearly higher than radiocommunication systems compliant with the GSM standard (for “Global System for Mobile”). They are thus indicated not only for vocal traffic but also for the transmission of images or digital videos.
In a radiocommunication system, each radiocommunications terminal comprises a transmission chain in order to be able to emit information in the radiocommunication network wherein it is provided to operate.
In relation with FIG. 1, a simplified diagram is shown of a conventional transmission chain 100 comprising a power amplifier 103.
More precisely, the transmission chain 100 comprises an analogue/digital convertor 101 (hereinafter designated by ADC 101) which transmits a control signal to an emitter 102. The emitter 102 generates an information signal (carrying the information to be emitted in the network) and transmits it to the power amplifier 103 so that the latter amplifies, before emission in the network via an antenna (not shown), the information signal. An output 104 of the power amplifier 103 is connected to a calibration point 105 which makes it possible to measure the output power of the power amplifier 103 during the calibration of the chain 100.
Concerning the control of the power of the amplifier 103 of the transmission chain 100, the GSM standard is much less restrictive than the UMTS standard. Indeed, for example, according to the GSM900 standard, the power servoing must take place in steps of 2 dB with a precision that ranges from +/−2 dB to +/−5 dB according to the power level between each step and this for an output power ranging between 33 dBm and 5 dBm while, according to the UMTS standard, the power servoing must take place in steps of 1 dB with a precision of +/−0.5 dB between each step and this for an output power ranging, for example, between +21 dBm and −50 dBm.
The architecture of the transmission chains compliant with the GSM standard is such that the frequency dispersion for a given power level is low. Indeed, these transmission chains do not include any band filter and the power amplifier 103 is conventionally a non-linear power amplifier which operates in compressed mode. In this case, in order to carry out the calibration it is sufficient to carry out a calibration at the central frequency and to apply it to all of the other frequencies of the band used.
Among the transmission chains compliant with the UMTS standard (hereinafter called UMTS transmission chains), first UMTS transmission chains use a fixed gain amplifier. These amplifiers are more linear than the amplifiers of GSM transmission chains as they are specifically polarised for this unique gain. It is therefore possible to calibrate such transmission chains over few frequencies and thus comply with the UMTS standard.
However, in order to reduce the energy consumption, second transmission chains are proposed that are compliant with the UMTS standard. These second transmission chains use a double gain amplifier which can be configured in such a way as to have a low gain or a high gain.
The calibration of these second transmission chains is complex due to the constraints imposed by the UMTS standard in particular in terms of precision on the step of the power servo of the transmission chain and due to the fact that the amplifier is a double gain amplifier.
The application of conventional calibration techniques of the first UMTS chains to these second UMTS chains does not procure sufficient precision (at frequencies other than those at which the calibration was carried out) in particular during the switching from the low gain configuration to the high gain configuration (or inversely) of the amplifier. Moreover, these conventional techniques are not very robust faced with the frequency dispersion of the transmission chain and are expensive, in particular in terms of memory (indeed, these conventional techniques imply the storage of a substantial amount of data).